Relative atomic mass definition

Note Care has to be taken when mass values from dated literature are cited. Prior to 1961 physicists defined the atomic mass unit [amu] based on Vie of the mass of one atom of nuclide 0. The definition of chemists was based on the relative atomic mass of oxygen which is somewhat higher resulting from the nuclides and contained in natural oxygen. 

You would never express the mass of a lump of gold, like the one in Figure 5.11, in atomic mass units. You would express its mass in grams. How does the mole relate the number of atoms to measurable quantities of a substance The definition of the mole pertains to relative atomic mass, as you learned in section 5.1. One atom of carbon-12 has a mass of exactly 12 u. Also, by definition, one mole of carbon-12 atoms (6.02 x 1023 carbon-12 atoms) has a mass of exactly 12 g. 

Gravimetric analysis is the process of converting an element into a definitive compound, isolating this compound from other constituents in a sample and then weighing the compound (Box 20.1). The weight of the element can then be calculated from the formula of the compound and the relative atomic masses of the elements involved. You need to be able to weigh accurately, by difference, a substance to four decimal places (see p. 23). 

It is possible to ask is there any definite interatomic distance (expressed in relation to the equilibrium distance) at which the chemical bonds are dissociated and the substance decays into free atoms To answer this question, let us start with the harmonic approximation. Because Bo = pc, where p is the density in g/cm, c the sound velocity in km/s, and Vo =A p (A is the relative atomic mass) Eq. 6.9 transforms into... 

Of the making of Periodic Tables there is no end. No version can ever be definitive because there are various incompatible objectives. Some authors provide a schematic version that is readable and easily reproduced, while others exploit devices such as the third dimension to express complexity. Some aim at simplicity or grace while others want to convey detailed information on such things as relative atomic mass, valency, electronic structure, melting and boiling points, electronegativity, radioactivity, metallic or non-metallic nature, geological affinities and so on. 

Water soluble protein with a relative molecular mass of ca. 32600, which particularly contains copper and zinc bound like chelate (ca. 4 gram atoms) and has superoxide-dismutase-activity. It is isolated from bovine liver or from hemolyzed, plasma free erythrocytes obtained from bovine blood. Purification by manyfold fractionated precipitation and solvolyse methods and definitive separation of the residual foreign proteins by denaturizing heating of the orgotein concentrate in buffer solution to ca. 65-70 C and gel filtration and/or dialysis. 

In 1808 John Dalton proposed his atomic theory, which included the statement that when atoms of two or more elements combine to form a compound, they combine in a definite ratio by number of atoms and by mass. This is called the law of definite proportions. This provided a means to determine the mass of one atom relative to another. It was necessary to assign a mass to one element to find the mass of another element in a compound. Today we use the most common carbon isotope, assigned a mass of 12.00 atomic mass units (amu), as the basis for comparative weights of the atoms. 

In Chapter 2, you learned that the mass of an atom is expressed in atomic mass units. Atomic mass units are a relative measure, defined by the mass of carbon-12. According to this definition, one atom of carbon-12 is assigned a mass of 12 u. Stated another way, 1 u = of the mass of one atom of carbon-12. 

The atomic mass of an element is the relative mass of an average atom of the element compared with which by definition has a mass of exactly 12 amu. Thus, since a sulfur atom has a mass 8/3 times that of a carbon atom, the atomic mass of sulfur is... 

The masses for the elements listed in the table inside the back cover of this text are relative masses in terms of atomic mass units (amu) or daltons. The atomic mass unit is based on a relative scale in which the reference is the C carbon isotope, which is assigned a mass of exactly 12 amu. Thus, the amu is by definition 1/12 of the mass of one neutral c atom. The molar mass of is then... 

All atomic nuclei are made up of protons and neutrons (known collectively as nucleons) the only exception is the lightest hydrogen nucleus, which consists of a single proton. The atomic number (Z) of an atom is the number of protons present in its nucleus (also the number of electrons in the neutral atom). The sum of protons (Z) and neutrons N) in a nucleus is referred to as the mass number ( 4). The mass number should not be confused with the atomic or nuclidic mass, which is the mass of the atom relative to that of a atom (which is, by definition, exactly 12.000... atomic mass tmits, amu). 

The modern definition of isotope refers to two or more forms of an element having different atomic masses. The original definition, by Soddy, was derived not from comparisons of atomic masses, but from studies of decay of chemically nonseparable, but clearly different, forms of the same radioactive elements. Thomson s 1907 experiment on canal rays passing through neon gas, described earlier, produced two parabolic paths, one corresponding to mass 20 and the other to mass 22. While this was the first evidence for isotopes of light, nomadioactive elements, the data were relatively crude and knowledge of atomic structure was insufficient to fully understand the result. 

The mass of an atom depends on the number of electrons, protons, and neutrons it contains and all atoms of a given isotope are identical in mass. The SI unit of mass (the kilogram) is too large to function as a convenient unit for the mass of an atom, thus a smaller unit is desirable. In 1961, the International Union of Pure and Applied Chemistry (lUPAC) defined the atomic mass unit (u) to be exactly equal to one-twelfth the mass of one carbon-12 atom. Carbon-12 ( C) is the carbon isotope that has six protons, six neutrons, and six electrons. Using this definition, we have that 1 u = 1.660539 X 10 kg. The atomic mass (sometimes called atomic weight) of an atom is then defined, relative to this standard, as the mass of the atom in atomic mass units (u). For example, the two naturally occurring isotopes of hefium, He and " He, have atomic masses of 3.01602931 u and 4.00260324 u, respectively. This means that a helium-4 (" He) atom is 4.00260324/12 = 0.33355027 times as massive as a carbon-12 atom. ... 

Avogadro s number is defined with respect to carbon-12—it is the number equal to the number of atoms in exactly 12 g of carbon-12. If Avogadro s number was defined as 1.00 X 10 (a nice round number), it would correspond to 1.99 g of carbon-12 atoms (an inconvenient number). Avogadro s number is defined with respect to carbon-12 because, as you recall from Section 2.6, the amu (the basic mass unit used for all atoms) is defined relative to carbon-12. Therefore, the mass in grams of 1 mol of any element is equal to its atomic mass. As we have seen, these two definitions together make it possible to determine the number of atoms in a known mass of any element. 

Based on the definition of the relative molecular mass Mr, it is seen that it corresponds to the sum of the masses of the constituent atoms A. The relative molecular mass Mr, often denoted molecular mass, is a non-dimensional quantity. 

The absolute mass of an isotope can be computed by summing the individual masses of the subatomic particles (see Table 2.1) and reporting the total number of grams that an isotope must weigh however, it is more convenient to quote masses in relative terms using the atomic mass unit (amu). By definition, the most abundant isotope of carbon is exactly 12.000 amu. All other isotope masses are reported relative to carbon-12. Appendix 1 lists the average atomic mass (i.e, the average of the abundances of the stable isotopes) of each element in atomic mass units. 

TOF spectra of the H atom products have been measured at 18 laboratory angles (from 117.5° to —50° at about 10° intervals). Figure 19 shows a typical TOF spectrum at the laboratory (LAB) angle of —50° (forward direction). By definition, the forwardness and backwardness of the OH product is defined here relative to the 0(7D) beam direction. The TOF spectrum in Fig. 19 consists of a lot of sharp structures. All these sharp structures clearly correspond to individual rotational states of the OH product, indicating that these TOF spectra have indeed achieved rotational state resolution for the 0(1D)+H2 — OH+H reaction. By converting these TOF spectra from the laboratory (LAB) frame to the center-of-mass (CM) frame... 

The three states are alike in that they all exhibit definite mass and volume under a given set of conditions. All consist of some combination of atoms, molecules or ions. The differences are stated above. Additional differences occur in their relative densities ... 

Spectral analysis shows quite clearly that the various types of atoms are exactly the same on Earth as in the sky, in my own hand or in the hand of Orion. Stars are material objects, in the baryonic sense of the term. All astrophysical objects, apart from a noteworthy fraction of the dark-matter haloes, all stars and gaseous clouds are undoubtedly composed of atoms. However, the relative proportions of these atoms vary from one place to another. The term abundance is traditionally used to describe the quantity of a particular element relative to the quantity of hydrogen. Apart from this purely astronomical definition, the global criterion of metallicity has been defined with a view to chemical differentiation of various media. Astronomers abuse the term metaT by applying it to all elements heavier than helium. They reserve the letter Z for the mass fraction of elements above helium in a given sample, i.e. the percentage of metals by mass contained in 1 g of the matter under consideration. (Note that the same symbol is used for the atomic number, i.e. the number of protons in the nucleus. The context should distinguish which is intended.)... 

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